Time variability of the mass-ejection phenomena associated with young stellar objects was identified a long time ago from studies of Herbig-Haro (HH) objects, which revealed the presence of knots and wiggling structures, and from measurements of their proper motion, which showed direct evidence for velocity variations [1]}. Close to the powering source(s) of the HH jet(s), chains of aligned knots are detected while at larger distances disconnected heads with lower velocities trace the impact against the ambient cloud. [2]} showed that velocity variability in the ejection process drives the formation of internal working surfaces, where the fast jet material catches up with the slow material, resulting into bright emission knots along the jet while the ejection direction variability produces a garden hose effect where the ejected material diverges from an equilibrium axis position.
Distribution of knot dynamical ages measured in Class I atomic jets suggests that there are up to three superposed ejection modes with typical timescales of a few \(10\hbox{\hspace{1.99997pt}yr}\) , \(10^2\hbox{\hspace{1.99997pt}yr}\)  and \(10^3\hbox{\hspace{1.99997pt}yr}\) , respectively [3]}.
